This invention relates to a power wrench intended for two-step tightening of screw joints.
In particular the invention concerns a power wrench of the type comprising a housing, an electric motor having a rotor, an output shaft coupled to said motor rotor, and a power supply means including a power converter with a variable frequency and voltage output and a current responsive shut-off means.
In power wrenches of the above type, there is a problem to avoid overtightening of so called stiff joints, i.e. screw joints with a steep torque growth characteristic in relation to angle of turn. By splitting the tightening process into two steps, of which the first step is a preliminary high speed step ending at a torque snug level and the second step is a final low speed high torque step ending at the desired tightening condition, there is avoided that the high speed inertia forces of the rotating parts of the wrench would cause an undesirable augmentation of the intended final torque level.
However, the two-step tightening process in itself, is not a guarantee that the desired final torque level is not exceeded when tightening very stiff joints. In such cases, it is required that even the first step is ended very rapidly to ensure that the inertia forces of the rotating parts of the wrench do not cause any augmented final torque level.
A problem inherent in previously known power wrenches is either that the interruption of the delivered torque is not fast enough and that with these power wrenches a torque overshoot is not avoidable when tightening very stiff joints or that extra wiring communication with the tool is needed for connection of speed sensing means on the tool to the power supply means.
In E.P. 271903 there is described an electrically powered two-step tightening apparatus in which the snug level is detected by a speed sensor mounted in proximity to one of the parts rotated by the motor. This sensor emits pulses as the motor rotates, and the frequency of these pulses is converted to a voltage which is reflective of the rotational speed of the motor. By means of a differential amplifier the derivative of the speed reflecting voltage is formed. A positive derivative indicates an acceleration of the motor speed, and a negative value of the derivative indicates a retardation. After the snug level has been reached, the motor speed is reduced and the tightening is completed at a lower speed.
This known apparatus has a less favourable feature in that the speed sensor arrangement requires extra wiring for communication with the control and power supply unit. This will make hand held tool versions more awkward to handle and more exposed to damage on the part of the wiring and, consequently, the snug level detecting function as well.
A solution to the wiring problem that might seem feasible is to provide a speed sensor in the form of a motor frequency sensitive means incorporated in the power supply unit and to have decelerations of the motor speed calculated. However, this is not a fast enough method to accomplish a snug level signal, because when for example using an asynchronous induction motor, there is a slip between the driving frequency and the motor speed. This slip, which is a sort of "lost motion", would cause an undesirable delay in the frequency change signal and, accordingly, a late interruption of the torque delivery.
For accomplishing a fast reacting power shut-off initiating means intended for interrupting the first, high speed tightening step, the present invention employs a retardation responsive activating means of a type previously used in impact wrenches. See for instance U.S. Pat. No. 2,768,546.
The present invention utilizes a known device including an electric power wrench supplied with power from a power converter with variable voltage and frequency output, and current responsive means are comprised in the power converter to accomplish a power shut-off as a desired final tightening level is reached. This belongs to prior art in itself and is marketed by Atlas Copco, the Assignee of the present application the name: Tensor A-CC-Drive.